Nowadays, the truss jib of a crane is made up of many jib sections. In general the cross-section of the jib section is a rectangle or other polygons. According to the different design requirements for a crane, the cross-section of the jib section is usually different. In order to use the jib sections having different dimensions of the cross-section on the same jib, usually two jib sections having different dimensions of the cross-section are connected each other by using transition section, as shown in FIG. 1 and FIG. 2. However, the above mentioned structure leads to the change of dimensions of the cross-section of the jib, thus it will affect the strength and the overall stability of a crane jib structure, and it will make some security problems of operation of the crane.